1. Field of the Invention
The present invention relates to a charged-particle beam system and, more particularly, to an aberration corrector and method of aberration correction capable of correcting higher-order aberrations.
2. Description of Related Art
Spherical aberration that cannot be corrected with a cylindrical symmetrical lens can be corrected using multipole lenses. Since established, this technology has been rapidly introduced into many practical apparatus. In recent years, excellent practical data derived using electron microscopes (TEMs and STEMs) equipped with aberration correctors have been vigorously published.
First, a phenomenon in which fifth-order spherical aberrations are created from spherical aberrations in a spherical aberration corrector and in an objective lens is described. FIG. 1 is a diagram illustrating generation of the fifth-order spherical aberrations from spherical aberrations in the spherical aberration corrector and in the objective lens.
In V. Beck, Optik, 53, 241-255 (1979), it is stated that if there is an extra optical distance L between a spherical aberration corrector 1 and a plane at which a correction is made (front or back focal plane of an objective lens 2) as shown in FIG. 1, extra fifth-order spherical aberration (C5) is introduced because the position of the electron beam is shifted at the correction plane. In this paper, the fifth-order aberration is described as issues produced when the spherical aberration corrector 1 is fabricated.
More specifically, because of shift of the position of the electron beam, angles δ1 and δ2 given by the following Eqs. (1) and (2) are produced.
                              δ          1                =                              C            s                    ·                                    r              3                        /                          f              4                                                          (        1        )                                          δ          2                =                                            r              2                        f                    +                                                    C                3                2                            ⁢                              r                2                3                                                    f              4                                                          (        2        )            r2 of the objective lens 2 is given by Eq. (3).r2=r1+Cs·r3·L/f4  (3)Therefore, from Eqs. (1)-(3), a relationship given by the following Eq. (4) is derived.
                                          δ            1                    +                      δ            2                          =                                            r              2                        f                    +                                                    3                ⁢                                                                  ⁢                                  C                  3                  2                                ⁢                L                                            f                2                                      ·                                          r                1                5                                            f                6                                              +                      higher            ⁢                                                  ⁢            order            ⁢                                                  ⁢            terms                                              (        4        )            For example, the extra fifth-order spherical aberration (C5) is introduced in the higher-order terms of Eq. (4).
A modern spherical aberration corrector using transfer lenses is free from the above-described problem because the distance L can be set to 0 by means of the transfer lenses.
A method of correcting spherical aberration using two hexapole fields is now described by referring to FIG. 2, which illustrates a method of correcting spherical aberration using the hexapole fields. In the above-cited reference, Beck also proposes a method of correcting spherical aberration by the use of two hexapole fields. According to Beck, two thin hexapole elements are combined to produce a negative third-order aberration (negative spherical aberration).
FIG. 2 shows that a negative spherical aberration given by Eq. (5) is produced by the combination of thin hexapole elements H1 and H2 which are spaced from each other by a distance of L.δ1x=+H(x12−y12) δ2x=−H(x22−y22) δ1y=−2Hx1y1 δ2y=+2Hx2y2 x2=x1+δ1xL  (5)y2=y1+δ1yL δ1x+δ2x=−2LH2(x13+x1y12)+higher order termsδ1y+δ2y=−2LH2(y13+y1x12)+higher order terms
Eq. (5) gives an example of correction of spherical aberration using a combination of aberrations, though A. V. Crewe and D. Kopf, Optik, 55, 1-10 (1980) states that a negative spherical aberration was created even from a single hexapole element.
JP2001-51613 states a method of removing deformation αn of an image in an electron optical system. Off-axis image deformations αnγm of order n+m which act identically on the deformation αn are corrected by moving or tilting the beam passage in the direction of the optical axis until compensation of the deformation of the image on the optical axis is completed. Furthermore, in this method, first-, second-, and third-order deformations of the image on the optical axis are corrected by correcting third-order deformation of the image on the optical axis in an electron optical system equipped with hexapole elements.
The aforementioned spherical aberration (in terms of order of geometrical aberration) is the third-order aberration. Even if this is corrected, the target resolution cannot be obtained under the condition where the other aberrations are left. Therefore, it is important to obtain a technique for correcting residual higher-order aberrations. Furthermore, in recent years, even electron microscopes have been required to be made up of a less number of components. Of course, the aberration corrector is required to be made up of a less number of components.
In the method disclosed in the above-cited JP2001-51613, a correction is made while moving or tilting the beam passage in the direction of the optical axis. Consequently, there is the possibility that labor is required to make adjustments for correcting both an aberration to be corrected and residual aberrations.
Especially, in order to correct higher-order aberrations, it is desired that low-order elements be combined, the number of components be reduced, and the correction be made quickly.